Using a 555, this configuration appears elsewhere on this site. This is from American documentation from the 90s. The indicator can be an LED or some device controlled by a relay.
This circuit was found in a 1969 Mullard publication. The transistor may be the modern equivalent...
This is the name given to the 1969 Electronics World voltage inverter. It is a series converter...
Operating at just 1.35 V this circuit was found in a 1969 Electronic Design. The circuit makes use...